1. Field of the Invention
The present invention is directed to rotary cutting tools. More particularly, the present invention concerns an improved cutting insert that is more easily and cheaply manufactured or modified for use, and which permits smaller inventories to be stored.
2. Description of the Prior Art
Rotary cutting tools involve the use of round cutting inserts which are generally shaped as a flat washer, and which serve as the cutting surfaces of the rotary cutting tool. A representative example of such rotary cutting tools is disclosed in U.S. Pat. No. Re. 26,637.
Characteristically, the cutting inserts are mounted on a base portion of a rotary cutting tool to rotate about their own central longitudinal axis when operationally placed in contact with a workpiece. The rotation of the inserts is caused by friction between the cutting inserts and the workpiece and is continuous for as long as the cutting tool and the workpiece are in contact with each other. The speed of rotation of the insert is controllable by adjusting the radial rake of the cutting inserts.
In general, increasing the radial rake of a cutting insert will serve to increase the speed of rotation of the cutting insert and increase the dissipation of heat attendant to operation of a rotary cutting tool. Higher speeds are usually involved when the rotary cutting tool is used with harder materials, whether milling, lathing or boring is being done.
Generally, the cutting insert is mounted on a cutting surface of a tool by the use of a bolt or the like, which extends through the center aperture of the insert, and which is fastened appropriately to the base of a rotary cutting tool. Commonly, a rotary cutting tool such as used for milling would be provided with several cutting inserts each of which is mounted individually to the base of the rotary cutting tool.
It is desireable to have the central aperture of each cutting insert situated to be concentric about the longitudinal axis of the insert. In the event that concentricity is lacking, the machined surface finish is rougher than is desireable, the resulting machined surface cutting plane is irregular, and chip formation and the rotation of the cutting insert is irregular which results in total vibration of the cutting tool as well as the workpiece, unequal cutting forces and varying dimensional tolerances. Any one of these conditions will shorten the cutting life of the cutting insert and can cause damage to the machined part, the machine or the cutting tool. As a general rule, the greater the error in concentricity, the more the above conditions will prevail.
The conventional cutting insert is manufactured by using extremely hard materials such as sintered tungsten carbide or titanium carbide. Due to limitations of the sintering process, it is extremely difficult to accurately place the central aperture to obtain absolute concentricity or to obtain a desired center configuration. As an example, yields of acceptable inserts using conventional techniques to fashion the center bore can be as low as twenty percent. Boring a hardened insert requires the use of diamond cutting tools, or the like, and is a slow, tedious and expensive process.
Because it is difficult to machine a central aperture in an insert blank (without a central aperture provided thereon) most users of rotary cutting tools have no choice but to simply purchase a substantial inventory of inserts with a variety of central aperture sizes. Such an inventory would further include cutting inserts having various thicknesses and outer diameters. Clearly, purchasing and maintaining substantial inventories of cutting inserts is expensive. It is notable that the current state of the art further makes impossible the fabrication of many more complex central aperture configurations.
Finally, it is important to note that when conventional cutting inserts are mounted and removed from the hardened steel stator or rotor with a rotary cutting tool, scratching or other damage to the stator or rotor often results due to the extreme hardness of the sintered carbide material used to form the insert.